Uneven Tire Pressures and Part Time 4WD Transfer Case ?

Lifted L5P

De Re Metallica
Mar 6, 2021
19
0
1
Montana
Got a question for ya’ll ... and hopefully this is the right forum for this question.

I know GM puts 60 psi front and 80 psi rear on the door sticker for heavy payload / towing considerations ... and possibly rear end stability as well.

However, we all know this causes the front tires to sag more than the rear when running with no payload / not towing due to a lower tire pressure and the heavier weight over the front tires (heavy diesel engine, etc.).

But this also causes a smaller rolling radius ... which causes the front and rear axles to turn at a slightly different speed.

How does a part-time 4WD transfer case driven by a chain between the two sprockets inside the transfer case deal with this difference ?

Is there enough give in all the drivetrain components that this small difference is not a major issue ?

One would think running a lower tire pressure in the rear thus trying to equal out the tire sag and the rolling radius would make more sense.

Or am I over-thinking this ?

Appreciate any cordial insights ...
 
Last edited:

DAVe3283

Heavy & Slow
Sep 3, 2009
3,415
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Boise, ID, USA
If it is the selectable 4WD, running in 4x4 on the street is never a great experience, regardless of tire pressure. Any serious turning and the drivetrain binds up. It won't hurt the drivetrain any, but will scrub the tires like crazy.

If it is the "auto" 4WD, which was not offered in the Duramax until the 2020 model year, then a clutch pack allows some slip at low throttle inputs to make it run smoothly on the street. But again, the main factor at play is the axle speeds when turning, not tiny differences in tire diameters from air pressure. (Effective circumference due to sag does change, IMO. Just not much.)

A coupe of us have retrofit the "auto" 4WD from the gas trucks into the Duramax, and that setup works pretty well. You can read about chrissun's build here, and I did virtually the same setup on both my rigs. It is pretty slick.
 
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2004LB7

Well-known member
Dec 15, 2010
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The difference in circumference is likely only from the tire stretch and that won't be much. Try chalking the tire and do one full revolution at 40 psi and 80 psi and tell us if you see any difference in distance
 
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Mike L.

Got Sheep?
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Aug 12, 2006
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You will only be using 4X4 in soft terrain with your part time TC so you will have tire slip and it won't matter. If you try and drive on hard surface no matter how perfect your tires match; you will break something. You are over thinking this.
 
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Lifted L5P

De Re Metallica
Mar 6, 2021
19
0
1
Montana
I wish there was an easy retrofit to the new transfer case with auto 4x4 (built-in clutch). Spend a lot of time on the highway in winter in the western US with intermittent ice patches ... would be nice to have the 4x4 option for those situations. Should have purchased a 2020 instead of my 2019 ... but I am leery of first model year trucks ... and the 2020’s weren’t available yet when I purchased my 2019. Oh well ... appreciate the response(s).
 

2004LB7

Well-known member
Dec 15, 2010
3,264
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I'm not seeing how 4wd is supposed to help with intermittent ice patches on the highway unless you are slowly trying to climb an incline, but I can't see this on a highway. And I'm sure you are aware but brakes are always 4x4 regardless of the 4wd setting
 

Lifted L5P

De Re Metallica
Mar 6, 2021
19
0
1
Montana
I'm not seeing how 4wd is supposed to help with intermittent ice patches on the highway unless you are slowly trying to climb an incline, but I can't see this on a highway. And I'm sure you are aware but brakes are always 4x4 regardless of the 4wd setting
When the accelerator is depressed in 2wd and you hit an ice patch (incline / flat terrain ... don’t matter) ... and the rear tire loses traction ... the rear locker kicks in ... and the rear end kicks sideways in a heart beat. Rear end kicking out sideways at highway speed is a bad thing. Seen many wrecked 2wd trucks that made the mistake coming thru this area that time of year without any / enough weight in the back of their truck to get decent traction. If you have ever driven the western US with black ice patches (you can’t see black ice due to black asphalt underneath) ... you will know exactly what I am talking about. Dangerous as sh-t. But I grew up in this area and been driving these roads for over 25 years ... and knock on wood ... haven’t wrecked a truck yet. Just miss the auto 4x4 setting in my 1500’s where I could leave it in the auto setting until I was thru the patchy ice highway ... it also had a limited slip but the auto 4x4 would kick in when I hit the ice patch and kept her straight.
 

Chevy1925

don't know sh!t about IFS
Staff member
Oct 21, 2009
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Phoenix Az
ill tell you the stabilitrac in your truck out-beats auto 4wd. Not only will it cut the throttle faster and sooner in 2wd than the auto 4wd could actuate, it will also keep you from going sideways at a much faster reaction before you may or may not notice something has happened. your rear locker also will not kick in if you are going over 25mph. the governor in side stops that action. if it is actually locking at that speed, something is broke inside.

at the end of the day, just dont overdrive the conditions of the road and keep the nanny features on, they work well. when it comes to stopping, nothing will fix that on black ice.
 

Chevy1925

don't know sh!t about IFS
Staff member
Oct 21, 2009
17,960
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Phoenix Az
on the subject of retrofitting a 2020 t-case into an older rig, i would bet the 15's to 19's can have that done. i dont have a clue where to start but the architecture between 19 and 20 is really similar from what i see.

the 2020 auto 4wd t-case is extremely robust and would be well worth putting in an older high hp dmax truck over the modded 246 auto 4wd.
 

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